Download The Birdseed Galaxy - Secondary Education

Galaxies and the Universe • H4
The Birdseed Galaxy
Activity H4
Grade Level: 6–12
Source: This activity was written by Marni Berendsen for the Night Sky Network project at the Astronomical
Society of the Pacific and adapted for the classroom by Andrew Fraknoi. This version is © 2010 by the
Astronomical Society of the Pacific. http://www.astrosociety.org Permission to use for any nonprofit
educational purpose (such as using it in one classroom) is freely granted. For any other use, please
contact the Society.
What’s This Activity About?
Tips and Suggestions
One of the hardest ideas for people to grasp is how vast
the Milky Way Galaxy is — how overwhelming its general size, shape, and organization is, and how small our
place in it turns out to be. This activity gives you some
simple tools to convey an understanding of these difficult concepts.
• The results of this activity are hard for students to absorb the first time they hear them, so be sure there is
some time set aside for discussion.
• Other activities which involve counting and estimating
the number of stars are G6, H3 and H6 on The Universe
at Your Fingertips disk.
• If you would like to show images of other spiral galaxies to the group, two places to find them are:
http://hubblesite.org/newscenter/archive/images/
galaxy/spiral/
http://www.noao.edu/image_gallery/spiral_
galaxies.html
What Will Students Do?
Students are given a scale factor, in which our solar system is about the size of their palm (or the size of two
quarters side by side). They have to estimate the size of
the Galaxy and then using a bird seed as the analog for
a star, estimate both how large a space 200 billion bird
seeds would take up (if rolled together) and how they
would be distributed in their scale model Galaxy.
What Will Students Learn?
Concepts
Inquiry Skills
Big Ideas
• Milky Way Galaxy
• Cosmic Scale
• Number of stars
• Modeling
• Comparing
• Imagining
• Measuring
• Inferring
• Describing
• Visualizing
• Scale and structure
• Models and simulations
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H4 • The Birdseed Galaxy
Galaxies and the Universe
The Birdseed Galaxy: A Scale Model of
our Milky Way
by Marni Berendsen
(Astronomical Society of the Pacific)
O
ne of the hardest ideas for people to grasp
is how vast the Milky Way Galaxy is — how
overwhelming its general size, shape, and organization is, and how small our place in it turns out to
be. This activity gives you some simple tools to convey
an understanding of these difficult concepts. It concentrates on our own Milky Way Galaxy and the objects
visible within it. When we say that the Orion Nebula is
1600 light years away or Polaris is 430 light years away,
this really means very little to students and audiences
because they have no standard by which to imagine how
far this is. This activity provides you with such a way of
seeing relative distances within our Galaxy.
Materials you will want to have:
• Two quarters
• A package of bird seed, available in any pet store.
Divide the birdseed into smaller packages, so that
each student group can have some. (Milo birdseed,
which is more round in shape, is especially useful.)
• Paper and pencil for participants to make their estimates on
Terms you will need to make sure your
students or audience understands are:
• Solar System — the Sun and its family: planets, dwarf
planets, moons, and smaller chunks
• Milky Way Galaxy — the great “island” (grouping) of
billions of stars in space, of which the Sun is only one
member
• Galaxy — any large group of billions of stars (we
know many billions of such galaxies)
• Universe — everything that our instruments can tell
us about (the collection of all the galaxies)
• Light Year — the distance that light travels in one
year (about 6 trillion miles or 10 trillion kilometers)
Some students or audience members
might not understand:
1. The difference between the Solar System, galaxy, and
universe.
2. That there is only one star in our Solar System: the
Sun. Many people believe stars are sprinkled among
the planets in the Solar System.
3. That our Sun is a star. It’s just a star that we are very
close to. The rest of the stars are tremendously far
away.
4. That the Solar System is within the Milky Way
Galaxy.
5. That the Milky Way Galaxy is so large that light —
the fastest thing in the universe — would take some
100,000 years to cross from one side of it to the other
Starting the Activity
1. Engage students or your audience in some discussion
about scale models and about the solar system. (If possible, do some of the scale model activities in Section D
of The Universe at Your Fingertips.)
For example, ask them if the Sun were the size of
a basketball, where do they think dwarf planet Pluto
would be? It would be a grain of sand, almost ¾ of a
mile from the basketball! And that’s just our home system — our local neighborhood. Now tell the group that
we want to enlarge our view and think about the Milky
Way Galaxy, the giant island of stars in which the Sun
is located. Tell them to be prepared to have their minds
boggled!
2. Engage students and audiences in a discussion about
the units we use to measure distances in space. While
miles or kilometers are fine for measuring distances on
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Galaxies and the Universe
H4 • The Birdseed Galaxy
Earth, they are just too
small to help much with
the huge gulfs of space
that separate us from the
stars.
If possible, do activities
D6 or G10 in The Universe
at Your Fingertips, but if
you haven’t, just get everyone talking about using light as your measuring stick. You could start
by defining a light minute,
the distance light travels
in one minute: it equals 18
million kilometers or 11
million miles. The Sun is
about 8 1/3 light minutes
from the Earth — that
means, light from the Sun
takes a little more than 8
minutes to travel to us.
Now, ask them to think
about a light hour, the
distance light travels in
an hour: its length is 1.1
billion kilometers or 670
million miles. Pluto is
Annotated diagram of the Milky Way Galaxy and its Spiral Arms of Stars, seen looking top down. (From NASA
about 5.5 light hours from
Spitzer Space Telescope Project)
the Sun.
Once we get out to the
100,000 light years across.
stars, even light (the fastest thing in the universe) takes
We’re going to shrink the Solar System in our minds,
years to travel the huge distances from Earth to there. So
so that the average distance from the Sun to Pluto,
we measure distances in light years, one of which equals
5½ light hours, is about the distance across a quarter.
9.5 trillion kilometers, or roughly 6 trillion miles. The
A quarter is one inch in diameter. So the diameter of
nearest star is about 4.3 light years away.
the Solar System (the distance from one side of Pluto’s
If your group feels overwhelmed by these numbers,
orbit to the other) would be about two quarters wide.
they now understand why we are doing this activity.
Everybody try to picture this. On this scale, the Sun is
Assure them that everyone feels overwhelmed by such
so small, you really couldn’t see it.
numbers.
NOTE to Teacher: On this scale, the Sun is 30 times
smaller than a grain of sand. The Earth is microscopic.
A Scale Model for the Galaxy
Since the palm of their hand is about the size of two quarNow tell the students or your audience that we will make
ters, you can also ask them to imagine that the solar sysa scale model that includes both our Solar System and
tem is shrunk down to be the width of a student’s hand.
our Galaxy. It’s not going to be easy, because the solar
Next tell them we will think about the Milky Way
system is only light hours wide and the Galaxy is about
Galaxy, the island of stars we live in. It is 100,000 light
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H4 • The Birdseed Galaxy
Galaxies and the Universe
YEARS across. Ask everyone to write down their guess
how big the Milky Way is on the scale where two quarters or a student’s palm represents the Solar System.
Would it be bigger than this room? The school? The
whole city?
Make sure everyone has a chance to write down a
suggestion before you give the answer. If there is time,
students might get together in small groups and compare answers. Each group could come up with a new
answer based on their discussion.
The answer is about 2,500 miles (4,000 km) across
on this scale. Ask the students or audience if they can
think of something they know that is about 2500 miles
wide. The answer is the continental United States from
coast to coast.
To summarize, if the Solar System is shrunk down to
a size about twice the size of a quarter, the Milky Way
Galaxy would span the width of North America! Give
everyone a chance to talk about this result with their
groups. Were they surprised?
again with their groups. On our scale model, 1000 light
years is about 25 miles (40 km). So our model Galaxy
would be 2500 miles across by 25 miles thick — a ratio
of about 100:1
Before you do the activity, find a city in your area that
is about 25 miles (40 km) away from where you are presenting this activity or two well-known local landmarks
that far apart. Alternatively, ask students to come up
with some well-known place about 25 miles away.
Then have them picture that the thickness of the
Galaxy is from here to there, straight up.
The cruising altitude of commercial airplanes is about
7 miles (11 km) up. The thickness of our scale model of
the Galaxy is about 3 times farther up than an airplane
flies. Ask students how many of them have flown in an
airplane? How small does an airplane that is seven miles
high look from the ground? How small would a quarter
look at that height? We couldn’t see it! So although the
Galaxy is very thin, its width is huge compared to our
solar system.
Further Characteristics of this Model
2. Where Are We in the Galaxy?
1. How Thick is the Galaxy?
Continue the discussion by asking, “Now that we know
how wide the Galaxy is, how thick do you think it is?”
Have students look at some images of spiral galaxies like
our own and then make an estimate.
Where we are, the Milky Way Galaxy is about 1000
light years thick. Ask them to estimate how thick that
would be on our model. If possible, have them discuss it
Remember that while the Galaxy is the size of our
country, two quarters are our Solar System. If we were
making a model of the Galaxy in the U.S. (and North
America), where in the country would we put the quarters representing the Sun’s family? Ask them to see if
anyone knows.
The Sun is about 27,000 light years from the center
of the Milky Way. Since the whole disk has a radius of
50,000 light years, you can say that we are roughly half-
The Milky Way arches across this 360-degree panorama of the night sky above the Paranal platform, home of European Southern Observatory’s Very
Large Telescope. The Milky Way stretches across the sky opposite the observatory. To the right in the image and below the arc of the Milky Way, two of
our galactic neighbours, the Small and Large Magellanic Clouds, can be seen. The open telescope domes of the observatory are all visible in the image,
which was made from 37 individual frames with a total exposure time of about 30 minutes, taken in the early morning hours. (ESO, H. H. Heyer)
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H4 • The Birdseed Galaxy
Galaxies and the Universe
way out from the center of our Galaxy. So where is that
in the U.S.? Maybe over the Rocky Mountains. What
else in North America is half-way from the center?
(Students could make measurements on a map of North
America, perhaps one they print out from the Web.)
3. The Number of Stars in the Galaxy
Our Galaxy is full of stars, only a tiny fraction of which
are visible to the naked eye or a pair of binoculars. There
are so many of them, it’s hard to count (and some are
hidden behind the vast reservoirs of dust which are part
of our Galaxy and block our view). So we have to estimate the number of stars in the Milky Way. Such estimates are partly based on surveys, much as pollsters
estimate support for a candidate or issue in the next
election based on representative surveys.
The best estimates of the number of stars in the
Galaxy range from 100 billion to over 400 billion. The
generally accepted best number these days is about 200
billion. This is a good time to ask the class or group: “Is
our Sun one of these billions of stars?” and to be sure
that everyone understands that it is.
So how many stars is 200 BILLION? Remember, on
the scale where the solar system is two quarters and
the Galaxy is the width of the US, the stars themselves
are tiny. So we need something tiny to represent them.
Let’s use bird seeds. (Pass out packages of bird seed to
each student group to let them get a feel for how big the
seeds are.) Mention that bird seeds are actually too big
for most of the stars on our North America sized galaxy
scale, but we’re just using them as an illustration.
Red milo bird seed
NOTE to Teacher: On this scale, the size of stars is
generally much smaller than the smallest grain of sand.
The bird seed is about the size of red giant stars. But for
the purposes of getting students to have a sense of scale,
this is an OK approximation.
How can we imagine what it would be like to have
200 billion bird seeds? Such numbers are very hard for
the human mind to deal with. Let’s start by building a
mental image of the volume taken up by 200 billion bird
seeds, if we just put them in one place. Ask students or
the audience what they think (they should be looking at
their package of bird seed while doing their estimate).
If they are stuck, you might ask: Would 200 billion bird
seeds fill their desk or their classroom? Would it be as
big as the gym in their school? Let them write down
their estimates, and, if there is time, discuss them with
their group.
Then ask, is there someone here about four feet (1.25
meters) tall? (If no one, indicate on your body how
high four feet is). Imagine a football field surrounded
by a wall four feet high. Fill the football field with this
birdseed to the top of the wall. That’s the volume of 200
billion seeds (representing the 200 billion stars in the
Milky Way Galaxy.) Ask students to take a minute and
compare this to their own estimates.
4. The Distribution of Stars in the Galaxy
Now, to complete our model, we need to put those birdseeds not on a football field, but scattered around North
America. How will they be scattered? Not evenly, since
the Milky Way Galaxy has more stars in the middle
than on its outskirts. Take about a third of the stars and
spread them over Kansas/Iowa for the central bulge of
our Galaxy. Take the rest and distribute them all over
North America, in a space 25 miles (40 km) deep.
Give students or the audience a bit of time to absorb
this while they look at their birdseed. The stars are very
small, and even though there are so many of them, the
space we have to put them in as very, very large.
So now ask the students: will the birdseeds be very
close to each other when we spread them out all over
North America? Not really. Are the stars very close to
each other in the Milky Way? No, they are separated by
huge gulfs of space. Let’s look at some real examples of
stars that students may have heard of or studied and put
them into our model.
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H4 • The Birdseed Galaxy
Galaxies and the Universe
On this scale our nearest star (the Alpha Centauri
system, 4+ light years away) is about 600 feet/200 meters or 2 football field lengths away. Sirius (8+ light years
away) is about 1/4 mile/400 meters away (4 football field
lengths). Polaris, the North Star, (430 light years away)
is about 11 miles/18 km away.
To review: if our solar system is the width of two quarters, the Galaxy is a disk as wide as the U.S., 25 miles
high, taking up most of the space in North America.
Our Solar System is about half-way out from the center of the Galaxy, maybe over the Rocky Mountains.
Imagine yourself again very, very tiny, flying high over
the Rocky Mountains. When you look straight up or
straight down, you see a just few stars. But look across
toward Kansas, and what do you see? Many, many stars,
fading into a haze as they get more distant, like distant
city lights fade into a haze. This is what the Milky Way in
the sky is: we are looking at our Galaxy edge-on in that
more crowded direction.
David Darling’s Milky Way Page: http://www.
daviddarling.info/encyclopedia/G/Galaxy.html
Quick Tutorial with Images by Prof. H. E. Smith:
http://cass.ucsd.edu/public/tutorial/MW.html
5. An Exercise for Students
Can you calculate how far from us in our model some
other well known objects in the Galaxy would be?
a. The double star Mizar in the handle of the Big
Dipper
b. The bright star Deneb
c. The bright star Vega
d. The Orion Nebula (M42) where new stars are seen
forming.
Answer: The Orion Nebula is about 40 miles (65
km) away (pick a city or landmark at that distance) and
would be a cloud of gas and dust ¾ of a mile (1 km)
wide. Compare that to the two quarters representing
our Solar System.
See the Calculation Details Sheet at the end for how
we figured out some of these analogies.
A Few Web Resources about the Milky
Way Galaxy:
Cornell University Milky Way Page (with Questions &
Answers):
http://curious.astro.cornell.edu/milkyway.php
Ten Interesting Facts about the Milky Way:
http://www.universetoday.com/22285/factsabout-the-milky-way/
The Universe at Your Fingertips • Astronomical Society of the Pacific
© copyright 2010 by the Astronomical Society
of the Pacific.
www.astrosociety.org
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H4 • The Birdseed Galaxy
Galaxies and the Universe
The Birdseed Galaxy: Calculation Details Sheet
© copyright 2010 Astronomical Society of the Pacific
Galaxy the size of North America: BREADTH = 100,000 light years
Sun to Pluto is represented by a quarter: 1" (3 cm) = 5.5 light hours
Diameter of Solar system: 11 light hours or 2" (6 cm)
How many inches or feet in a light year on this scale?
365 days in a year x 24 hrs in a day = 8760 hrs in a yr
÷ 5.5 light hrs in 1 inch (3 cm) = 1592 inches (40 meters) in a light yr
÷ 12 inches (30 cm) in a foot = 132 feet (40 m) for one light yr
How many feet or miles (m or km) across the Galaxy on this scale?
132 feet (40 m) for one light yr x 100,000 light years = 13,200,000 feet
÷ 5280 feet in one mile = 2,500 miles (4,000 km) for 100,000 light years
THICKNESS of the Galaxy: 1,000 light years
132 feet for one light yr x 1,000 light years = 132,000 feet
÷ 5280 feet in one mile = 25 miles (40 km) for 1,000 light years
RATIO: 2500 miles across by 25 miles thick – about 100:1
How much volume do 200 billion “birdseed stars” take up?
Use a 4" x 4" x 2" (10cm x 10cm x 5cm) box and a 35mm film canister as a scoop. Use a big bag of the milo birdseed.
Count the number of birdseeds in one scoop, then calculate how many “birdseed stars” will fit in the box, as follows:
Scoop up level scoopfuls of stars and count how many scoops it takes to fill the box. It should be about 14 scoops.
How many “stars” does a 4" x 4" x 2" box hold?
There are approximately 1300 “stars” in one scoop. A 4 x 4 x 2 box holds about 14 scoops.
1300 x 14 = ~18,000 stars in a box
So how many boxes do we need to fill up to get 200 billion stars? 200,000,000,000 ÷ 18,000
stars in a box = ~ 11 million boxes
A football field, goal line to goal line is 300 feet (91 m) and about 160 feet (50 m) wide or 3600" by 1920".
How many boxes in one layer? 3600" divided by 4" (width of box) = 900 boxes from goal line to goal line.
1920" divided by 4" (length of box) = 480 boxes sideline to sideline.
To cover the field one layer deep: 900 x 480 boxes = 432,000 boxes.
How many layers to make 11 million boxes (using our example above)?
11,000,000 boxes needed ÷ 432,000 boxes in one layer = ~ 25 layers of boxes
How high is 25 layers? 25 layers x 2" per layer = 50 inches or ~4 feet (1.25 m)
• Imagine football field surrounding by four walls and filled 4 feet deep with birdseed. That’s 200 billion seeds
representing the 200 billion stars in the Milky Way Galaxy.
• Now spread them 25 miles (40 km) deep all over North America, with about ⅓ of them on the middle over
Kansas/Iowa. Enjoy your trip!
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